1) Field of the Invention
The present invention relates to a semiconductor laser device and a semiconductor laser module suitable for an excitation light source for erbium doped fiber amplifiers (EDFA), Raman amplifiers or the like, and a Raman amplifier using the device or the module.
2) Description of the Related Art
Recently, with popularization of various multimedia such as the Internet, a demand of increasing the capacity of the optical communication is increasing. Conventionally, in the optical communication, it is common to transmit information by a single wavelength in a band of 1310 nm or 1550 nm, which has small light absorption by an optical fiber. With this method, it is necessary to increase the number of optical fibers to be disposed in a transmission path, in order to transmit a large quantity of information, thereby causing a problem in that the cost increases, with an increase in the transmission capacity.
Therefore, a dense-wavelength division multiplexing (DWDM) communication method is now being used. In this DWDM communication method, an EDFA is mainly used, to transmit information using a plurality of wavelengths in the 1550 nm band, which is the operation band thereof. With this DWDM communication method or the WDM communication method, since one optical fiber is used to transmit a plurality of optical signals of different wavelengths simultaneously, it is not necessary to lay a new line, making it possible to remarkably increase the transmission capacity of the network.
A common WDM communication method using this EDFA has been put into practical use from 1550 nm, where flattening of gain is easy, and recently, the band has been expanded to 1580 nm, which has not been used due to a small gain coefficient. However, since a low loss band of the optical fiber is wider than a band that can be amplified by the EDFA, the spotlight has been centered on an optical amplifier operated in the band outside the EDFA band, that is, the Raman amplifier.
While the gain wavelength range of an optical amplifier using rare earth ions such as erbium as a medium is determined by an energy level of the ion, the Raman amplifier has a feature that the gain wavelength range is determined by a wavelength of an exciting light, and an optional wavelength range can be amplified by selecting the wavelength of the exciting light.
With the Raman amplification, when a strong exciting light is shone onto the optical fiber, gain appears on the long wavelength side by about 100 nm from the exciting light wavelength, due to induced Raman scattering. When a signal light in the wavelength band having this gain is shone onto the optical fiber in this excited state, this signal light is amplified. Therefore, with the WDM communication method using the Raman amplifier, the number of channels of the signal light can be further increased, as compared with the communication method using the EDFA.
FIG. 22 is a block diagram which shows the construction of a conventional Raman amplifier used in the WDM communication device. In FIG. 22, semiconductor laser modules 182a to 182d which include Fabry-Perot type semiconductor light emission elements 180a to 180d and fiber gratings 181a to 181d respectively in pair, output a laser beam, which is the source of the exciting light, to polarization multiplexing couplers 61a and 61b. The wavelengths of the laser beams output from the respective semiconductor laser modules 182a and 182b are the same, but light having a different plane of polarization is multiplexed by the polarization multiplexing coupler 61a. Similarly, the wavelengths of the laser beams output from the respective semiconductor laser modules 182c and 182d are the same, but light having a different plane of polarization is multiplexed by the polarization multiplexing coupler 61b. The polarization multiplexing couplers 61a and 61b output the polarization-coupled laser beams respectively to a WDM coupler 62. The wavelengths of the laser beams output from the polarization multiplexing couplers 61a and 61b are different from each other.
The WDM coupler 62 couples the laser beam output from the polarization multiplexing couplers 61a and 61b through an isolator 60, and outputs the coupled laser beam to an amplification fiber 64 as the exciting light, through a WDM coupler 65. A signal light to be amplified is input from a signal light input fiber 69 through an isolator 63 to the amplification fiber 64 to which the exciting light has been input, and coupled with the exciting light and subjected to the Raman amplification.
The signal light subjected to the Raman amplification in the amplification fiber 64 (amplified signal light) is input to a monitor light distribution coupler 67 through the WDM coupler 65 and an isolator 66. The monitor light distribution coupler 67 outputs a part of the amplified signal light to a control circuit 68, and outputs the remaining amplified signal light to a signal light output fiber 70 as the output laser beam.
The control circuit 68 controls the light emission state, for example, optical power of each semiconductor light emission elements 180a to 180d based on the input part of the amplified signal light, and performs feedback control so that the gain band of the Raman amplification has a flat characteristic.
FIG. 23 is a diagram which shows the schematic construction of a semiconductor laser module using the fiber grating. In FIG. 23, this semiconductor laser module 201 has a semiconductor light emission element 202 and an optical fiber 203. The semiconductor light emission element 202 has an active layer 221. The active layer 221 is provided with a light reflection surface 222 at one end, and a light radiation surface 223 at the other end. The light generated in the active layer 221 is reflected by the light reflection surface 222 and output from the light radiation surface 223.
The optical fiber 203 is arranged on the light radiation surface 223 of the semiconductor light emission element 202, and optically coupled with the light radiation surface 223. In a core 232 of the optical fiber 203, a fiber grating 233 is formed at a predetermined position from the light radiation surface 223, and the fiber grating 233 selectively reflects light of a characteristic wavelength. That is, the fiber grating 233 functions as an external resonator, and forms a resonator between the fiber grating 233 and the light reflection surface 222, and the laser beam of a specific wavelength, selected by the fiber grating 233, is amplified and output as an output laser beam 241.
In the semiconductor laser module 201 (182a to 182d), however, since a distance between the fiber grating 233 and the semiconductor light emission element 202 is long, relative intensity noise (RIN) increases due to resonance between the fiber grating 233 and the light reflection surface 222. With the Raman amplification, since the process in which amplification occurs comes early, if the intensity of the exciting light is fluctuated, the Raman gain also fluctuates. This fluctuation of the Raman gain is output directly as the fluctuation of the amplified signal intensity, causing a problem in that stable Raman amplification cannot be performed.
Here, as the Raman amplifier, there are a backward pumping system in which a signal light is excited from the rear side, like the Raman amplifier shown in FIG. 22, as well as a forward pumping system in which a signal light is excited from the front side, and a bi-directional excitation system in which a signal light is excited bi-directionally. The one mainly used as the Raman amplifier at present is the backward pumping system. The reason is that the forward pumping system in which the weak signal light progresses in the same direction together with the strong exciting light has a problem in that the excited optical power fluctuates. Therefore, it is desired to develop a stable excitation light source also applicable to the forward pumping system. That is to say, if a semiconductor laser module using the conventional fiber grating is used, there is a problem in that the applicable excitation system is limited.
The above-described semiconductor laser module 201 needs to optically couple the optical fiber 203 having the fiber grating 233 and the semiconductor light emission element 202. Since it is a mechanical optical coupling in the resonator, there is the possibility that the oscillation characteristic of the laser may change due to mechanical vibrations, causing a problem in that stable exciting light cannot be provided.
The Raman amplification in the Raman amplifier is based on a condition that a polarization direction of the signal light coincides with a polarization direction of the exciting light. That is to say, the Raman amplification has a polarization dependency of the amplified gain, and it is necessary to reduce the influence caused by a deviation between the polarization direction of the signal light and the polarization direction of the exciting light. In the instance of the backward pumping system, the signal light has no problem since the polarization becomes random during propagation. In the instance of the forward pumping system, however, the polarization dependency is strong, and it is necessary to reduce the polarization dependency by means of cross polarization multiplex, depolarization or the like of the exciting light. That is, it is necessary to reduce the degree of polarization (DOP).
Since the Raman amplification is used for the WDM communication method, the amplified gain characteristic may be changed corresponding to the number of wavelengths of the input signal light, and hence high-output operation having a wide dynamic range is required. In this instance, however, fine fluctuations actually occur in the driving current dependency of a monitor current, causing a problem in that stable optical amplification control becomes complicated or difficult. The monitor current is a current obtained when an optical output leaked from the rear end of the semiconductor laser device is received by a photodiode (PD).
For example, FIG. 24 is a diagram which shows the optical output dependency (Lo) of the monitor current (Im). In the optical output dependency of the monitor current shown in FIG. 24A, when exceeding a certain optical output, the monitor current becomes wavy with an increase in the optical output, which shows that fluctuations have occurred. In this instance, since the optical amplification control of the semiconductor laser device is performed based on the monitor current, the correspondence with the optical output becomes complicated, and as a result, the optical amplification control becomes also complicated. On the other hand, in the optical output dependency of the monitor current shown in FIG. 24B, when exceeding a certain optical output, the monitor current increases stepwise with an increase in the optical output. In this instance, the optical amplification control of the semiconductor laser device becomes unstable, since it is performed based on the monitor current.